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Occurrence of the Gasoline Additives MTBE and BTEX Compounds in Urban Stormwater in the United States, 1991-95

Thomas J. Lopes, Gregory C. Delzer, and John S. Zogorski U.S. Geological Survey, 1608 Mountain View Rd., Rapid City, SD 57702
Phone (605) 355-4560 ext. 240, Telecopier (605) 355-4523,

Methyl tert-butyl ether (MTBE) is the most common oxygenate added to gasoline. Oxygenates such as MTBE increase the gasoline's oxygen level and are added in an effort to decrease vehicular carbon monoxide emissions and ozone levels in the atmosphere. MTBE is soluble in water, is less biodegradable than common gasoline compounds, such as benzene, toluene, ethylbenzene, and total xylene (BTEX), and was the second most frequently detected volatile organic compound (VOC) in a national study of shallow urban ground water. Urban stormwater is a possible source of MTBE in shallow ground water. The U.S. Geological Survey sampled stormwater in 16 cities and metropolitan areas that are required to obtain permits from the U.S. Environmental Protection Agency (USEPA) to discharge stormwater from their municipal storm-sewer system into surface water. Five hundred ninety two stormwater samples, collected in these cities and metropolitan areas during 1991 through 1995, were analyzed for 62 VOCs including MTBE and BTEX compounds.

MTBE was the seventh most frequently detected VOC in these stormwater samples, following toluene, total xylene, chloroform, total trimethylbenzene, tetrachloroethene, and naphthalene. MTBE was detected in 6.9 percent of stormwater samples and, when detected, concentrations ranged from 0.2 to 8.7 micrograms per liter (g/L), with a median of 1.5 g/L. All detections of MTBE were less than the USEPA's draft lifetime health advisory (20 g/L) for drinking water. Eighty-three percent of all detections of MTBE in stormwater were in samples collected during October through March, which corresponds with the expected seasonal use of oxygenated gasoline in areas where carbon monoxide exceeds established air-quality standards. The median concentration of MTBE and benzene for all samples was statistically higher in samples collected during October through March than samples collected during April through September. Sixty-six percent of all MTBE detections occurred with BTEX compounds, and a linear increase in concentrations was found when these compounds occurred together. The linear increase could indicate a common source of MTBE and BTEX for those samples. Toluene and total xylene were the most frequently detected VOCs. Detected concentrations of toluene and total xylene ranged from 0.2 to 6.6 g/L and 0.2 to 15 g/L with median concentrations of 0.3 and 0.4 g/L, respectively.


1997, American Society of Testing and Materials Symposium on Envireonmental Toxicology and Risk Assessment, 7th, St. Louis, Mo.

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